This method relates to a method of pressure treatment for metal billets that have specified physical and mechanical properties, which are derived from their fine grain structural characteristics. The method relates to treating rods, bars and other particularly long billets.
It is known that physical and mechanical properties, for example strength and plasticity, depend on a material""s microstructure. Therefore, changes to the microstructure characteristics may change these properties. For example, it is usually necessary to create a cellular or sub-grain microstructure within the material to strengthen a material.
Large physical and mechanical changes of a material""s properties may be achieved by refining the micro-crystal grains, for example those grains that are sized from 10 to 0.1 xcexcm. These materials, when compared with coarse grain materials, exhibit significantly higher strength characteristics. At higher temperatures these materials exhibit low flow pressure and higher levels of plasticity, or even super-plasticity. In order to form a micro-crystal structure, it may be necessary to use more radical deformation techniques than those used to create other types of fragmented microstructures, for example sub-grain microstructures.
Known billet deformational treatment methods comprise equal-channel angular extrusion and pressure torsion. These methods are used to produce billets with specified physical and chemical characteristics, because the methods produce known micro-crystal structures. These methods also permit the development of small mass and sized fine-grain billets. These methods are, however, highly labor and energy intensive.
Another metal and alloy deformational treatment method comprises deformation that is done by multiple reduction steps and is generally followed by an increase in cross-section by extrusion and upsetting. This method permits development of small sized rods from soft materials, in which the rods have smaller physical and chemical properties.
Billet treatment involves significant energy consumption because of the force required to overcome friction that is generated when the surface of the machine tool and billet meet to overcome hydro-static pressure created during extrusion, for example extrusion using backwater. This method is generally unsuitable for the production of large-sized billets, for example billets in the form of rods that are five to six meters in length with a 150-200 mm minimal diameter formed from a hard to deform alloy. In this situation, it is necessary to use an apparatus including a press that can develop up to several tens of thousands of ton pressure as well suitable machine tools. Typically rods with a micro-crystal structure or grains sized from 3 to 8 xcexcm and a diameter of 30 to 40 mm are produced by multi-operational forging or rolling methods from billets with an initial diameter of 400 mm or larger.
The method reduces the cost of deformational treatment for long rods and large diameter billets that require a specified internal microstructure, including micro-crystal microstructure, and specified physical and mechanical properties. This specified internal microstructure may be achieved using various treatment methods, including deformation of a billet section through reduction of a billet""s cross-section. Reduction of the cross-section uses tools, for example a roller, that permit movement along and across the billet""s axis and as well being spinning the billet about its surface. At least one support stand can be used for correct positioning and placement of the billet. Additionally, a pre-specified strain level can be achieved using a deformation that includes one of torsion, upsetting, and drawing, that uses machines such as the above-discussed stand. The stand applies a specified deformation to the billet at the deformed, or strained, section at a pre-specified temperature. This deformation obtains a specified microstructure with intrinsic physical and mechanical characteristics.
It is recommended that reduction of the billet""s cross-section, using a hot-metal roller, be done by applying pressure along the billet""s axis using stands and clamps; reducing the billet""s cross-section by laterally moving the billet through rollers; reducing the billet""s cross-section by longitudinal and lateral moving the billet through rollers; reducing the cross-section by moving the billet through rollers that the rotational axes create crossed angles with the billet""s axis; reducing the billet by passing the billet through rollers located at 120xc2x0 from each other; and choosing a length of the treated section not to exceed three minimal diameters of the rod""s reduced cross-section. Further, the reduction of the billet""s cross-section, which uses a hot-metal roller, may be done while applying torsion using stands and rollers; applying reverse torsion; and applying deformation of the billet""s cross-section using a figure roller. The roller profile includes several sections, including a middle section that includes the largest cross section; an intermediate section on both sides of the middle section that have the smallest cross-section; and two end sections.
Alternatively, the method for billet reduction using a hot-metal roller, is by reducing a cross-section through compression along the billet""s longitudinal axis; or allowing upsetting the billet after passing the billet through rollers about its lateral axis, in which the length of movement is not greater than the amount of lateral deformation for this section during reduction); allowing the billet""s upsetting after longitudinal and lateral rolling about the billet""s axis; upsetting the billet while rolling under the following condition:
"sgr"u greater than "sgr"i less than "sgr"e,
where "sgr"i is the level of stress on the strained section, determined by considering the deformation resistance produced by the rollers during rolling, and "sgr"u is the stress caused by loss of the billet""s stability, and "sgr"e is the stress caused by compression of the billet""s undeformed sections.
Furthermore, the reduction of the billet""s cross-section comprises rolling using a hot-metal roller while continuously and consecutively deforming the length of the billet under treatment; and treating sections of the billet in which the distances between each section are not greater than 3 times the diameter of the billet after treatment.
For a billet formed of single-phase alloys, the deformation method includes deformation is done with a real strain amount is not less than 3, and in particular with a strain amount of 1.4, with the strain rate of 101-10xe2x88x922sxe2x88x921 at a temperature of (0.3-0.5) Tmelt), where Tmelt is melting point. For billets formed of a multi-phase alloy, deformation is done with a real strain amount of not less than 3, in particular with a strain amount of 1.4, and with a strain rate of 10xe2x88x921-10xe2x88x924sxe2x88x921 at a temperature of (0.5-0.85) Tmelt, where Tmelt is melting point.
For titanium alloy billets that comprise a lamellar structure, the deformation method, with a real strain amount of not less than 3, comprises reduction or applying torsion with rolling and upsetting that occurs simultaneous the rolling. These steps are done in addition to the reducing, upsetting, and deforming at 700xe2x88x92Ta.t. and a strain rate 10xe2x88x921-10xe2x88x924sxe2x88x921, where Ta.t. an allotropic transformation temperature. For sections of titanium billet with a lamellar structure, the method is done with not less than 1.1 times reduction of the billet cross-section at temperature of Ta.pxe2x88x92Ta.p.+(10-50), followed by a coil step at a rate of not less than 1xc2x0/s, and applying torsion and upsetting at a temperature not higher than 700xe2x88x92Ta.p. and strain rate of 101-10xe2x88x924sxe2x88x921.
For heat resistant nickel billets, the method includes deforming at temperatures not higher than the temperature of complete dissolution of the xcex3xe2x80x2-phase, deforming each billet section within 10%-20% of the specified temperature and strain amount, so as not to a change in the stress flow of more than 5%-10%; and deforming each billet section at a specified temperature and strain amount xcex6 until changes of the strain amount do not increase of the coefficient of the sensitivity rate m=(log N1xe2x88x92log N2)/(log "xgr"1xe2x88x92log "xgr"2) Up to amounts of 0.3-0.8; in which N1 and N2 are pressure amounts (moment, pressure of compression, or extension at the corresponding deformation) that are applied to the billet, both before and after the change of the strain amount from the amount "xgr"1 to the amount "xgr"2.